About 200 North American Eared Grebes (Podiceps nigricollis
californicus) at Tule Lake Refuge in northern California were observed
engaging in successive waves of mass pattering and pattering flights on
25 May 2011. Most grebes present in a part of a canal were involved in
this activity. Counts of grebes on the morning of 26 May suggest an
important portion of the Eared Grebes seen in pattering could have left
the area over night. The behavior was characterized as zugunruhe.
Directed mass pattering of Eared Grebes may contribute to
synchronization of the onward migration of the birds involved.

North American Eared Grebes (Podiceps nigricollis californicus) are
seldom seen in flight, except when they migrate (Bent 1919, Gaunt et al.
1990). The migration of the species has been well studied (Storer and
Jehl 1985, Gaunt et al. 1990, Jehl 1997, Cullen et al. 1999, Jehl and
McKernan 2002, Jehl and Henry 2010). Cullen et al. (1999) indicate
migration flights begin around dusk and end before dawn. Jehl and Henry
(2010) note strict correspondence of departure with near-total darkness.
Grebes tend to gather as the time for departure nears (Jehl and McKernan
2002). Predeparture activities include group diving, and submerging and
surfacing in near unison. A unique call is given as grebes prepare to
depart and immediately before actual take-off (Jehl and Henry 2010).

Daytime flights are possibly observed only when grebes rebuild
their flight muscles prior to migration when they may perform one or two
short practice flights (Jehl and Henry 2010) or race across the surface
in short practice flights, often in small groups (Jehl and McKernan
2002). I was surprised to observe a mix of pattering and flight by
larger groups of Eared Grebes in Northern California during daylight
conditions. I describe these common pattering flight maneuvers and
discuss their possible meaning.

METHODS

A study of courtship of Eared Grebes was undertaken at Upper
Klamath Lake, Oregon, and Lower Klamath Refuge and Tule Lake Refuge,
both in northern California, from 14 to 27 May 2011. This region is
known to support thousands of Eared Grebes each year for nesting, water
levels permitting. The California refuges hosted 7,397 and 3,700 nests,
respectively, in 2003 and 2004 (Shuford et al. 2006). Fieldwork was from
0700 to 1700 hrs each day using a car as a blind. The car was parked at
suitable places along roads near bodies of water and remained immobile
for up to 3 hrs. The behavior and displays of grebes were documented
either by photograph, video film or immediate voice recording. All
observations of pattering flights are from Tule Lake Refuge, part of the
Klamath Basin National Wildlife Refuges, an artificial water impoundment
of mostly open water covering ~5,200 ha at an altitude of 1,200 m and
surrounded by croplands. The observations were in an area called the
English Channel (41[degrees] 51' 202 N, 121[degrees] 29' 727
W) in the central part of the wildlife tour into the refuge. This is an
L-shaped canal, <50 m in width. It opens at its northern end into
large sump lA, an open and shallow area of the lake. It takes a left
turn after ~1.6 km in a straight line from north to south (NS canal or
NS part of the English Channel) and continues east for another 0.5 km
(EW canal or EW part of the English Channel) until ending at a dam-levee
that separates it from the adjacent larger sump 1B (Fig. 1). The entire
canal is devoid of emerging vegetation.

I differentiate between pattering (a grebe with flapping wings runs
with paddling feet or even partially glides over the water surface, but
remains in constant contact with the water), pattering flight (after an
initial pattering, a grebe is airborne for a distance limited to a few
meters during which it does not touch the water surface), and real
flight (the distance covered while airborne exceeds 10 m). It is well
established that Eared Grebes use pattering in the retreat display and
during escape/pursuit or more generally during aggression (McAllister
1958, Cullen et al. 1999); these occurrences are not included. My
objectives in this paper are to provide a full description of pattering
and pattering flights by larger numbers of grebes, and to discuss
possible reasons for their occurrences.

[FIGURE 1 OMITTED]

RESULTS

Observations in the southern English Channel on 25 May started at
0900 hrs. Over 200 Eared Grebes were scattered partially in loose groups
all over the EW part of the English Channel around midday when about
three quarters of them engaged in pattering. The grebes did so in
consecutive waves, all into a western direction towards the connection
to the NS canal. The sudden take-off by one or two grebes seemed to
cause others in their immediate vicinity and on their way to move in the
same direction. Groups of 10-30 birds pattered over a short distance
(20-30 m), some briefly loosing contact with the water surface in a
pattering flight. Grebes getting briefly airborne possibly did so to
avoid collision with conspecifics that remained stationary on the water
surface. Grebes landed ahead of others that started similar maneuvers in
their wake, perhaps carrying along some of those that had just stopped
pattering. A few additional waves of pattering were launched. Some birds
dived after landing; others elevated their necks, remained alert, and
looked around without changing their westward orientation. Most of the
population, including subgroups closer to the NS canal which were not
observed to patter, was swimming in the direction of the NS canal. The
eastern and central parts of the EW canal were rather empty of Eared
Grebes after some 2-3 rain, leaving only a few Westem (Aechmophorus
occidentalis) and Clark's grebes (A. clarkii) and a few ducks
remaining. Fewer than 100 Eared Grebes were still swimming in the
western part of the EW canal towards the connection with the NS canal
when they encountered about 40 birds swimming in a group to reenter the
EW canal. A rough count less than 10 min later indicated that >200
Eared Grebes had again spread over this canal.

Pattering and pattering flights started anew only ~20 min after the
start of the first general movement by the Eared Grebes. Take-off by one
or two Eared Grebes incited others in their surroundings to join as
before. The birds moved westward in several waves and continued swimming
into the same direction after landing. More grebes left the EW canal
where only about 30 remained, all towards its western end. A first group
of swimming grebes returned ~1 min later. It was followed by other loose
groups. I counted 130 grebes 5 rain later and soon >240 birds were
again present inside the EW canal.

A longer period without group pattering, but with continuous
calling, occasional displays and much surface feeding on phantom midges
(Chaoborus crystillinus) followed until ~1400 hrs. Individual grebes
performed feeding dives, but no group diving, or submerging and
surfacing in near unison was observed. The general pattering in waves
and westward swimming towards the connection with the NS canal started
again and most Eared Grebes finally left the EW canal. The first grebes
had turned and swam to return to the EW canal when a sudden simultaneous
eastward pattering of >50 re-entering grebes occurred. Two or three
more waves by other groups followed immediately. Five minutes later, 232
grebes were counted inside the EW canal.

Only the continuous and contiguous calls of the birds were heard
for ~20 min. Ten birds then initiated a fourth round of pattering in
waves. This time, the grebes had no common general direction. The grebes
more in the central part of the observed area moved towards the dam,
those already closer to the eastern end pattered into a more
southwestward to westward direction. The population present divided into
two groups. About 100 grebes were clustered near the dam and another 100
were scattered over the upper western third of the EW canal. The space
in between both groups remained mostly empty. The western group started
immediately to swim eastward while the eastern group slowly dispersed.
The groups soon melted and spread over the empty space that had
separated them.

Perhaps five additional pattering flights of up to 4-5 grebes were
observed in between the different mass pattering and pattering flights.
It was not known whether these were premature attempts to initiate a
wave or whether they were unrelated to the mass movements.

The observations ended at ~1700 hrs and 257 grebes were counted in
the EW canal (26 in the connecting corner square to the NS canal), 65
were present in the lower half of the NS canal and 347 in the upper
half. Only five additional Eared Grebes were detected at the mouth to
sump 1A. Other parts of the sump close to the English Channel were empty
of Eared Grebes. A count of the birds at 0700 hrs on the following day
totaled exactly 400 individuals, 269 less than the previous count. Only
77 grebes were observed inside the EW canal (28 in the connecting corner
square) while the NS canal had 323 grebes. Three hours later, 126 Eared
Grebes were recorded in the EW canal and 337 in the NS canal. The two
counts on 26 May revealed quite differing numbers of grebes. The EW
canal held 131 to 180 grebes less and the entire English Channel held
206 to 269 grebes less than on the afternoon of 25 May.

DISCUSSION

Eared Grebes had arrived at Tule Lake Refuge in the course of the
previous 2-3 weeks. I assume that shortly after arrival, their wing
muscles were still in good flight condition on 25 May and intense
practicing could not have explained the mass pattering. Most birds were
actively courting, but the group pattering did not appear to be related
to pair bonding. There is also no reason to believe the grebes tried to
divert an aerial predator with common flight activity as several
instances of Bald Eagles (Haliaeetus leucocephalus) appearing in flight
over the grebes or even trying a catch in the canal did not trigger much
reaction. Birds pattering to escape a pursuing conspecific or to flee
possible danger incited alarm at the most to a handful of other Eared
Grebes in their immediate vicinity. The generalized pattering by larger
groups of Eared Grebes observed appeared unrelated to courtship,
aggression, fear or predator presence. A similar or comparable behavior
by Black-necked Grebes (P. n. nigricollis) in Europe has not been
reported.

There is comparable agitation in Silvery Grebes (P. occipitalis)
during migration towards breeding areas. Fjeldsa (1982) noted that
Silvery Grebes show high restlessness and form long lines that move back
and forth on a lake from where, in the subsequent night, at least part
of the population departed. He termed this pre-migratory restlessness.
Movements of a group of 70 Silvery Grebes at Laguna Las Encadenadas,
Argentina, in December 2006, were not limited to swimming, but included
sudden quasi-simultaneous take-offs of individuals more at the rear end
of the line. Some flew up, reaching a height of ~2 m, possibly to avoid
collision with the birds preceding them. They landed again in front of
the group that was moving in one direction. The grebes at the rear end
acted similarly. The group changed direction as it approached the shore,
but continued swimming in a line, and pattering and flying from the back
to the front (Konter 2009).

Eared Grebes at Tule Lake Refuge all swam actively into the same
direction, although they did not form one line. They showed pattering
and pattering flights in waves and repeated the directed group
movements. A priori the comparison of total counts of grebes inside the
English Channel on the following day strongly suggests at least a major
portion of the population had left the area. Zugunruhe seems an
appropriate characterization for the Eared Grebes' behavior.
Additional pre-departure activities at Tule Lake Refuge including group
diving, submerging and surfacing in near unison as reported by Jehl and
Henry (2010) were not obvious. The grebes' diving and swimming
seemed to be predominantly related to feeding, except the dives after
mass pattering involved only a minority of a group. Active vocalization
may have helped group cohesion, but it could not be distinguished from
advertising by solitary birds or from contact calling by partners
momentarily separated.

It is not known to where the departing grebes flew and whether they
targeted breeding areas in the region or flew a long distance. Eared
Grebes can move to other sites used for breeding, even after arrival in
a breeding area, or emigrate from the region (Cullen 1998). It is also
unknown whether the grebes departed in flocks from the English Channel
and whether they headed in one or different directions. I assume they
were migrants and the extent of their pattering flight maneuvers
suggests an eagerness to move on.

The counts of 25 and 26 May show that not all Eared Grebes had left
the English Channel over night. Grebes present in the NS part were not
observed on 25 May and they may not have engaged in group pattering. The
first count on 26 May showed that low numbers of grebes were present
inside the EW canal and the higher later count suggests that new grebes
were continuously settling there. Over 200 Eared Grebes left the English
Channel during the night and this number corresponds as an order of
magnitude to the numbers involved in the group pattering. Thus, most
pattering grebes could have left over night and it is likely their
zugunruhe contributed to a simultaneous departure. They were gradually
replaced by conspecifics moving into the EW canal on the following day.

Eared Grebes often do not arrive within a short lapse of time
inside a breeding region where numbers generally build up over several
weeks. They synchronize, however, nest establishment (McAllister 1956,
Boe 1994). In this context, it is of interest to further investigate how
a conspicuous pre-migratory group pattering as observed at Tule Lake
Refuge may contribute to a coordinated onward flight inside a breeding
region that would facilitate simultaneous colony establishment by large
numbers of pairs. Unfortunately, the data from Tule Lake Refuge do not
permit any conclusion to be drawn.

ACKNOWLEDGMENTS

I am grateful to Michele Nuss from the Tule Lake Refuge
Headquarters who was of great help in the preparation of my fieldwork. I
thank J. R. Jehl Jr and C. E. Braun for critical review and constructive
comments on the first draft.

Received 13 July 2011. Accepted 19 September 2011.

LITERATURE CITED

BENT, A. C. 1919. Life histories of North American diving birds,
Order Pygopodes. U.S. National Museum Bulletin 107:1-47.